1. Field of the Invention
The present invention relates to a blazed grating device and to a diffraction grating device, and particularly to a light separation device that separates light according to the properties of the light. The present invention also relates to an illumination optical system provided with such a blazed grating device, diffraction grating device, or light separation device.
2. Description of the Prior Art
In an image display apparatus that modulates illumination light with a spatial modulation device so that the modulated light represents an image, various kinds of optical devices are used to direct the illumination light to the spatial modulation device. In cases where a liquid crystal display (LCD) is used as a spatial modulation device, the LCD needs to be fed with linearly polarized light that is polarized uniformly on a fixed polarization plane (i.e. the plane on which electrical vectors vibrate). For this reason, when a light source that emits unpolarized light is used, a polarization separation (PBS) prism or polarizing plate is used together to extract linearly polarized light that suits the LCD.
FIG. 45 shows the structure of a PBS prism. A PBS prism is composed of a PBS film 151c sandwiched between two prism elements 151a and 151b, each having the shape of a rectangular equilateral triangle in cross section. The PBS film 151c separates P-polarized and S-polarized light components by selectively transmitting one and reflecting the other. However, in general, at angles of incidence smaller than about 45°, the PBS film 151c does not exhibit sufficient selectivity between transmission and reflection, and thus does not offer satisfactory separation. This is the reason that a PBS film 151c is usually sandwiched between two prism elements 151a and 151b so as to be used in the form of a PBS prism having the shape of a square prism.
Simply separating differently polarized light components as achieved with a PBS prism, or simply absorbing an unnecessary linearly polarized light component with a polarizing plate, results in the loss of about half of the illumination light fed from a light source. To avoid this, it is customary to integrate together the two linearly polarized light components obtained as a result of polarization separation by rotating the polarization plane of one light component through 90° with a half-wave plate so that the polarization plate of this light component coincides with that of the other (for example, as disclosed in Japanese Patent Application Laid-Open No. H10-197827). Performing polarization conversion in this way helps almost eliminate the loss of illumination light, and thus makes it possible to illuminate a spatial modulation device with high light use efficiency.
In cases where a reflective LCD is used, since the optical path of the illumination light that illuminates the LCD coincides with the optical path of the light reflected from the LCD, it is necessary to separate the illumination light beam and the reflected light beam, of which the latter represents an image, somewhere in their optical paths. To achieve this, an optical device that reflects one and transmits the other of the illumination and reflected light beams is used.
A PBS prism as described above is used for this purpose also. In cases where a reflective LCD is used in such a way that the light that has its polarization plane rotated through 90° by being modulated by the LCD represents an image, if the light transmitted through a PBS prism is used as illumination light, the light that represents the image is reflected from the PBS prism; alternatively, if the light reflected from the PBS prism is used as illumination light, the light that represents the image is transmitted through the PBS prism. In either case, it is possible to direct the reflected light, which represents the image, in a direction different from the direction leading to the light source.
FIG. 46 shows another optical device used to separate illumination light and reflected light. This optical device has a large number of grooves 152d, each having a V-shaped cross section, formed in the top surface 152a of a transparent flat plate 152, and is arranged with its bottom surface 152b facing a reflective LCD 153. The light fed from a light source is introduced into the flat plate 152 through an end surface 152c thereof, and then travels inside the flat plate 152 by being totally reflected from the top and bottom surfaces 152a and 152b. Meanwhile, the light strikes the surfaces of the grooves 152d and is reflected therefrom. As a result, the light is then transmitted through the bottom surface 152b, and then illuminates the LCD 153. The light reflected from the LCD 153 enters the flat plate 152 through the bottom surface 152b, and then exits from the flat plate 152 by being transmitted through the top surface 152a. 
Between the flat plate 152 and the LCD 153, a polarizing plate 154 is disposed to form the illumination light into linearly polarized light. The LCD 153 is so controlled that, not the linearly polarized light component that has its polarization plane rotated through 90° by being modulated, but the linearly polarized light component of which the polarization plane has not been rotated by modulation is used as light representing an image.
One conventional way to display color images is to provide each pixel of an LCD with a color filter that selectively transmits red (R), green (G), or blue (B) light. However, in this arrangement, two-thirds of the white light fed from a light source is lost by the color filters, which results in low light use efficiency. To avoid this, in recent years, it has been becoming increasingly common to separate illumination light into R, G, and B light components that travel along slightly different optical paths and provide an LCD with a microlens array so that the R, G, and B light components strike different pixels.
FIG. 47 shows an optical system used to separate colors by this method. This optical system is composed of three dichroic mirrors 155R, 155G, and 155B. The dichroic mirrors 155R, 155G, and 155B selectively reflect R, G, and B light components, respectively, and transmit the light components of the other colors. The dichroic mirrors 155R, 155G, and 155B are arranged at an angle to one another so as to reflect the light incident thereon in different directions. The differences between the angles at which the reflected R, G, and B light components travel are twice as great as the differences between the angles at which the dichroic mirrors 155R, 155G, and 155B are arranged.
As shown in FIG. 48, the LCD 153 is provided with a microlens array 156 that is so arranged that each of the microlenses 156a constituting it faces three adjacent pixels 153R, 153G, and 153B. Each microlens 156a receives the R, G, and B light components from different directions and makes them converge on different pixels 153R, 153G, and 153B. In this way, it is possible to direct the whole light fed from the light source to the pixels of the LCD 153, and thereby obtain bright images.
A device called a digital micromirror device (DMD) having a large number of mirror elements arranged in a two-dimensional array is also used as a spatial modulation device. In a DMD, the angle of each mirror element is variable so that, according to this angle, the light incident thereon is reflected selectively in one of two predetermined directions. Of the light thus reflected in two directions by the DMD, the portion reflected in one direction is extracted as light representing an image, and the portion reflected in the other direction is discarded as unnecessary light.
FIG. 49 shows a typical optical system used to illuminate a DMD. This optical system is composed of two prisms 157a and 157b arranged with a minute gap between them. The light fed from a light source is introduced into the prism 157a from the side. The surface 157c of the prism 157a that faces the prism 157b is so formed that the introduced light strikes it at an angle of incidence grater than the critical angle, and therefore the introduced light is totally reflected from the surface 157c. As a result, the light is then transmitted through the surface 157d, and then illuminates the DMD 158. The light reflected from the DMD 158 enters the prism 157a through the surface 157d. The light that has entered the prism 157a reaches the surface 157c at an angle of incidence smaller than the critical angle, and is thus transmitted therethrough. The light is then transmitted through the prism 157b. In this way, an optical system used to illuminate a DMD exploits total reflection on and transmission through prism surfaces.
A polarizing plate is easy to use because it has a simple structure and has the shape of a flat plate. However, the transmittance that a polarizing plate exhibits to the linearly polarized light component that is transmitted therethrough is about 80% at best. This causes loss of light. Moreover, a polarizing plate absorbs all light components other than the linearly polarized light component that is transmitted therethrough. This makes the polarizing plate hot and thereby affects the other devices arranged nearby such as an LCD. When intense light is used to obtain bright images, the polarizing plate becomes particularly hot.
A PBS prism does not absorb light, and therefore does not become hot. Moreover, a PBS prism permits the use of both the light transmitted therethrough and the light reflected therefrom. Moreover, a PBS prism can easily be made to offer a transmittance or reflectance of substantially 100%, and thus excels in light use efficiency. However, a PBS prism has a thickness that is equal to the width of the entrance surface thereof, and thus makes the display apparatus that incorporates it larger and heavier. In a display apparatus of a projection type that forms an image on a screen by projecting light representing the image onto the screen, using a PBS prism makes the back focal length of the projection optical system longer, and therefore a large projection optical system is required to obtain bright images. This problem of making display apparatuses larger and heavier also applies to the optical system shown in FIG. 49, which, too, uses prisms.
The optical device shown in FIG. 46 is easy to use because it has a simple structure and can be produced simply by forming grooves in a flat plate. However, part of the light that has entered the flat plate by being reflected from an LCD is reflected from the surfaces of the grooves, and thus cannot be transmitted through the flat plate. As a result, dim stripes appear in the displayed image. Such dim stripes can be made less conspicuous to a certain degree by making the width of the grooves narrower, but there is no fundamental remedy for this problem.
The optical system shown in FIG. 47, composed of three dichroic mirrors, excels in light use efficiency. However, those dichroic mirrors are separate devices, and therefore it is difficult to arrange them at correct angles to one another. Thus, assembling the optical system requires a long time and lowers the overall manufacturing efficiency.
Diffractive optical devices are used in various fields of optics. A diffractive optical device has a grating surface with microstructures formed thereon that consist of minute projections and depressions arranged in a periodic pattern, and deflects light by diffraction. Diffractive optical devices are grouped into a bi-level type having projections and depressions, both with flat surfaces, respectively formed at two different levels (heights), a multi-level type having one or more intermediate levels between such projections and depressions, and a blazed type having slanted surfaces so as to have a sawtooth-shaped section. Diffractive optical devices of any of these types may be of a transmission type that diffracts the light that is transmitted therethrough, or of a reflection type that has its grating surface coated with a reflective film so as to diffract the light that is reflected therefrom. Transmission-type diffractive optical devices often have their grating surface coated with an anti-reflection film to obtain higher transmittance.
Though not a diffractive optical device, a Fresnel lens also has a large number of minute slanted surfaces, and is thus formed as a blazed device having a sawtooth-shaped section. In diffractive optical devices, the difference between the levels of projections and depressions is about equal to the wavelength of light, so that light is diffracted. By contrast, in Fresnel lenses, the difference between the levels of projections and depressions is several times or more as great as the wavelength of light, so that light is deflected exclusively by refraction.
Diffractive optical devices and Fresnel lenses have the great advantage of being thin optical devices.
According to the Japanese Patent Application Laid-Open No. H10-197827 mentioned previously, a polarization separation device is built as a diffraction grating formed out of an isotropic transparent material, and an optically anisotropic layer formed out of a birefringent material, and the like. However, even when a diffraction grating is formed out of an isotropic transparent material, unless due consideration is given to the thickness of the diffraction grating, the diffraction grating may exhibit birefringence, of which the effect can lower light use efficiency. Moreover, diffraction gratings are optical components having microstructures, and therefore it is difficult to achieve high reliability in a diffraction grating by forming it as a single member. In addition, diffraction gratings are required to be easy to produce in terms of their moldability.